Also, we don't really know how long Kovu spent at the pridelands between the time he was accepted and when Zira attacked Simba. Obviously, the film couldn't show everyday he was there, so I think they used the two key scenes- "Upendi" and "training" to show how Kiara and the general atmosphere of the pridelands changed him.There might have been several other days, maybe even a month, and Kovu was then able to see a batter way of life, but the directors/producers cut it down to two key scenes.
Also Nuka, Vitani and Zira checked up on him, as we saw, and I don't think they would check up on him everyday, maybe once a week.
I think that if this is true, if Kovu was with Kiara in the pridelands for longer than shown, he wouldv'e been able to make the transition easier, and not so quickly.
Also, I agree with those who feel that kovu was his "true self" when he was with Kiara, not when he was a soldier to Zira.
But, you know, thats my opinion
